Transformerless conversion concept based on storage exchange for future fault tolerant and galvanic insulated interconnection of grids

Abstract

The paper proposes a modular conversion concept with integrated energy storage for transferring energy between any combination of AC grids, DC grids and modular renewable parks whose sources need to be close to earth potential whereas the grids they feed feature much higher voltages. By relying on pairs of detachable energy storage elements featuring properly sized capacity, the conversion concept provides high-voltage galvanic insulation, high fault tolerance and simple frequency conversion among all grids without using any transformer. Each pair of energy storage elements is associated to a couple of AC/DC or DC/DC units constituting voltage source cells. In turn, each phase of a grid is structured modularly as proper parallel and series connections of such cells. The pivotal idea is to transfer energy by periodical exchange of the storage elements among the cells, while always avoiding any simultaneous galvanic contact among them. This simple principle assures galvanic insulation and high fault tolerance since the grids or renewable sources never exchange energy directly. The energy exchange is always indirect and mediated by the energy storage elements. Owing to this property no fault is propagated immediately among the interconnect grids and, until sufficient energy is present in the storage elements, one gains precious time for enterprising much better emergency actions in the grids, actions that would be otherwise impossible with the nowadays much reduced time scales. The exchange of storage elements by reconnection can rely on contacts, like the ones for high-voltage tap changers, and on different moving means, like Robots, if physical displacement is envisaged. This latter realization broadens the concept considerably because the storage elements can be transported, via autonomous vehicles, for the long distances which divide the energetically connected entities. A specific reconnection sequence of the storage elements assures their disconnection and reconnection at zero current, thereby maximizing the lifespan of the switches. Although the present state of the art in storage technologies renders the concept still uneconomical and unfeasible today, the ongoing technological advancements could open future scenarios one should be prepared to embrace. This paper is not about a conventional solution for today. It is about highlighting what could be futuristically possible in exchanging large amounts of energy among grids, and/or renewable sources, when material science will provide storage technologies with sufficiently high capacity and number of cycles.